Auto merge of #530 - berkowski:baud_constants, r=alexcrichton

Added baudrate constants.

Addresses #528, adding baudrate constants for `bsd` and `notbsd` flavors of `unix`.

Passes `libc-test` locally on ubuntu 14.04 LTS with the additional (uncommitted) entries into in `build.rs`:

```
cfg.skip_const( move |name| {
        match name {
            # ... snip ...
            "PTRACE_O_SUSPEND_SECCOMP" |
            "CLONE_NEWCGROUP" |
            "NETLINK_LIST_MEMBERSHIPS" |
            "NETLINK_LISTEN_ALL_NSID" |
            "NETLINK_CAP_ACK" |
            "PR_CAP_AMBIENT_CLEAR_ALL" |
            "PR_CAP_AMBIENT_LOWER" |
            "PR_CAP_AMBIENT_RAISE" |
            "PR_CAP_AMBIENT_IS_SET" |
            "PR_CAP_AMBIENT" |
            "PR_FP_MODE_FRE" |
            "PR_FP_MODE_FR" |
            "PR_GET_FP_MODE" |
            "PR_SET_FP_MODE" |
            "PR_MPX_DISABLE_MANAGEMENT" |
            "PR_MPX_ENABLE_MANAGEMENT" |
            "PR_GET_THP_DISABLE" |
            "PR_SET_THP_DISABLE" |
            "PR_SET_MM_MAP_SIZE" |
            "PR_GET_MM_MAP_SIZE" |
            "PR_SET_MM_MAP" |
            "NLM_F_DUMP_FILTERED" |
            "EPOLLEXCLUSIVE" => true,

            _ => false,
        }
    });
```
I'm assuming this is because I'm stuck using `linux-libc-dev:3.13.0-24.46` for the moment and those constants are defined in newer versions.